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POPULAR SCIENCE REVIEW. 
■which so closely resembles an Ascidian in its development, has a perivisceral 
cavity which essentially corresponds with the atrium of the Ascidian, though 
it is formed in a somewhat different manner. One of the most striking 
peculiarities in the structure of Amphioxus is the fact that the body wall 
(which obviously answers to the somatopleure of one of the higher Verte- 
brata, and incloses a ‘ pleuro-peritoneal ’ cavity, in the walls of which the 
generative organs are developed) covers the branchial apertures, so that the 
latter open into the ‘pleuro-peritoneal’ cavity. This occurs in no other 
vertebrated animal. Kowalewsky has proved that this very exceptional 
structure results from the development of the somatopleure as a lamina 
which grows out from the sides of the body, and eventually becomes united 
with its fellow in the middle ventral line, leaving only the so-called ‘ respi- 
ratory pore ’ open. Stieda has mentioned the existence of the raphd in the 
position of the line of union in the adult animal. Rathke described two 
‘abdominal canals’ in Amphioxus; and Johannes Muller, and more recently 
Stieda, have described and figured these canals. However, Itathke’s canals 
have no existence, and what have been taken for them are simply passages 
or semi-canals between the proper ventral wall of the abdomen and the in- 
curved edges of two ridges developed at the junction of the ventral with the 
lateral faces of the body, which extend from behind the abdominal pore 
where they nearly meet, to the sides of the mouth. Doubtless, the ova 
which Kowalewsky saw pass out of the mouth had entered into these semi- 
canals when they left the body by the abdominal pore, and were conveyed 
by them to the oral region. The ventral integument, between the ventro- 
lateral laminae, is folded, as Stieda has indicated, into numerous close-set, 
longitudinal plaits, which have been mistaken for muscular fibres, and the 
grooves between these plaits are occupied by epidermic cells, so that, in 
transverse section, the interspaces between the plaits have the appearance of 
glandular coeca. This plaited organ appears to represent the Wolffian duct 
of the higher Vertebrata, which, in accordance with the generally embryonic 
character of Amphioxus, retains its primitive form of an open groove. The 
somatopleure of Amphioxus, therefore, resembles that of ordinary Vertebrata 
in giving rise to a Wolffian duct by invagination of its inner surface. But 
the Wolffian duct does not become converted into a tube, and its dorsal or 
axial wall unites with its fellow in the raphe of the ventral boundary of the 
perivisceral cavity.” 
The true Position of the Sponges is, as pointed out by Mr. A. S. 
Packard in the “ American Naturalist ” (Feb. 1875), between the Coelente- 
rata and the Protozoa. The embryo sponge arises from eggs which undergo 
a total segmentation of the yolk. The free swimming larva later in its life 
becomes fixed, loses its external cilia, but retains its cellular walls, now 
composed of two layers, which are supported by silicious or calcareous 
needles or spicules developed in the inner layer. To regard such an 
organism as a Protozoan, or even to compare it with a compound Radiola- 
rian such as Sphserozoum, with its silicious spicules and aggregations of one- 
celled organisms, would not seem warranted. We have, in fact, in the 
light of the anatomical investigations of Lieberkiihn, Carter and Clark, and 
the combined anatomical and embryological studies of Haeckel, Metschnikoff, 
and Carter, no grounds for leaving them among the Protozoa. Indeed, one 
